DOI: 10.1148/rg.284075157
RadioGraphics 2008;28:1005-1017
© RSNA, 2008
Neoplastic and Non-neoplastic Proliferative Disorders of the Perirenal Space: Cross-sectional Imaging Findings1
Venkateswar R. Surabhi, MD2,
Christine Menias, MD,
Srinivasa R. Prasad, MD,
Ankitkumar H. Patel, MD,
Arpit Nagar, MD, and
Neal C. Dalrymple, MD
1 From the Department of Radiology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Dr, San Antonio, TX 78229 (V.R.S., S.R.P., A.H.P., A.N., N.C.D.); and Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Mo (C.M.). Presented as an education exhibit at the 2006 RSNA Annual Meeting. Received July 11, 2007; revision requested September 11, 2007, and received January 4, 2008; accepted January 30. All authors have no financial relationships to disclose.
Address correspondence to S.R.P. (e-mail: prasads{at}uthscsa.edu).
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Abstract
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The perirenal space, located between the anterior and the posterior renal fasciae, is shaped like an inverted cone with an apex that extends into the iliac fossa. Perirenal tumors and pseudotumors primarily originate either from the kidney or as part of a systemic disease process and have characteristic histopathologic features and biologic behavior. The lesions may be classified on the basis of their distribution and imaging features as solitary soft-tissue masses (renal cell carcinoma, lymphangioma, hemangioma, and leiomyoma), rindlike soft-tissue lesions (lymphoma, retroperitoneal fibrosis, and Erdheim-Chester disease), masses containing macroscopic fat (angiomyolipoma, liposarcoma, myelolipoma, and extramedullary hematopoiesis), and multifocal soft-tissue masses (metastases, plasma cell tumors). Because of overlap in imaging findings among these diverse perirenal lesions, a definitive diagnosis in most cases can be established only at histopathologic analysis. However, an imaging pattern–based approach may facilitate the diagnosis and optimal management of perirenal tumors and pseudotumors.
© RSNA, 2008
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LEARNING OBJECTIVES FOR TEST 2
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After reading this article and taking the test, the reader will be able to:
- Describe the spectrum of neoplastic and nonneoplastic proliferative conditions that may involve the perirenal space.
- Identify the imaging features of perirenal tumors and pseudotumors and correlate them with histopathologic findings.
- Define a pattern-based approach to diagnostic imaging of perirenal tumors and pseudotumors.
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Introduction
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The perirenal space is a retroperitoneal space that is limited anteriorly by the anterior renal fascia (Gerota fascia) and posteriorly by the posterior renal fascia (Zuckerkandl fascia). These two fasciae fuse to form the lateroconal fascia laterally and blend loosely with the periureteric connective tissue medially (1,2). Superiorly, the two fasciae are fixed to the diaphragmatic fascia above the adrenal glands; inferiorly, they blend with the iliac fascia. The anterior and posterior renal fasciae enclose a gradually tapering conelike space produced by the embryologic ascent of the kidneys from the pelvis to the adult retroperitoneal position (1). The perirenal space is divided into multiple compartments by thin fibrous lamellae and bridging septa (Kunin septa) that connect the renal capsule with the anterior and posterior renal fasciae (3). The perirenal space abuts the bare area of the liver on the right and the subphrenic space on the left. The anterior renal fascia, which overlies the upper portion of the right kidney and adrenal gland, may be deficient and allow communication of the perirenal space with the hepatic bare area (4). The left and right perirenal spaces communicate with each other across the midline and with the pelvic retroperitoneal spaces below the iliac fossa. The perirenal space contains the kidneys, adrenal glands, proximal ureters, perirenal fat, lymphatic vessels, and blood vessels (1) (Fig 1).
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Figure 1. Axial contrast-enhanced CT scan in a patient with acute pancreatitis shows the cross-sectional anatomy of the left perinephric space (PNS), with its anterior limit (A) marked by the Gerota fascia and its posterior limit (P) by the Zuckerkandl fascia. LC = lateroconal fascia.
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The perirenal space is commonly involved in a wide variety of neoplastic and nonneoplastic conditions (Table 1). Perirenal tumors and pseudotumors may arise from the kidney or as part of a disease process that affects multiple organs. In this article, we describe the imaging features of a heterogeneous group of perirenal lesions and propose an imaging pattern–based approach to their characterization (Table 2).
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Solitary Soft-Tissue Masses
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Renal Adenocarcinoma
Renal adenocarcinomas are the most common malignant neoplasms of the kidneys. They have variable histologic features, biologic behavior, and imaging findings (5).
Renal adenocarcinoma with perinephric spread constitutes the most commonly occurring perirenal soft-tissue mass. The clear cell variant is the most frequently seen at histologic analysis (approximately 60%–80% of cases). Clear cell carcinoma typically is hypervascular but shows heterogeneous enhancement at computed tomography (CT) and magnetic resonance (MR) imaging (5) (Fig 2). Other subtypes, such as papillary (10%–15% of cases) and chromophobe (5%–10% of cases) carcinomas, are typically hypovascular and show relatively homogeneous contrast enhancement. Accurate preoperative staging of renal adenocarcinoma permits optimal patient management. In addition, local staging is helpful for determining the best surgical approach and procedure (laparoscopic versus open incision). Whereas stage T2 tumors are resected by using an anterior approach, stage T3 tumors are resected with a retroperitoneal approach. Contrast-enhanced CT and MR imaging have high sensitivity, specificity, and accuracy for staging of renal adenocarcinomas and detection of their extracapsular spread (6,7). In a study of 15 patients with Robson stage I renal carcinoma, Catalano et al (7) found 96% sensitivity, 93% specificity, and 95% accuracy for the diagnosis of perirenal fat invasion based on the depiction of hyperattenuating streaks and soft-tissue nodules on CT scans. However, precise delineation of perinephric fat invasion and differentiation of the renal capsule from the tumor pseudocapsule may be difficult in some cases.
Malignant Fibrous Histiocytoma
Malignant fibrous histiocytoma is the most common soft-tissue sarcoma in adults. Approximately 15% of cases occur in the retroperitoneum (8). The lesions typically are characterized by areas of spindle cells arranged in a storiform pattern and by pleomorphic areas with haphazardly arranged sheets of fibroblasts and histiocytes. However, malignant fibrous histiocytoma is a heterogeneous disease with distinct histologic subtypes. In addition to the storiform-pleomorphic subtype (the most common), myxoid, giant cell, inflammatory, and angiomatoid variants have been described in the literature (9).
At imaging, retroperitoneal malignant fibrous histiocytoma appears as a large heterogeneous mass that contains areas of necrosis and hemorrhage (Fig 3). The mass frequently displaces the kidney and other retroperitoneal structures. Calcification is seen in 7%–20% of retroperitoneal malignant fibrous histiocytomas (8,10,11).
Renal Infections
Although renal infections are not proliferative disorders, some infectious conditions (eg, focal pyelonephritis and xanthogranulomatous infection) may be mistaken for neoplastic disorders of the kidney and the perinephric space. Acute pyelonephritis may result either from an ascending infection secondary to a urinary tract obstruction or from hematogenous dissemination. Patient demographics and clinical and laboratory findings are frequently diagnostic of acute pyelonephritis. However, CT usually is performed to detect underlying predisposing factors, confirm the diagnosis of acute pyelonephritis in atypical cases, and assess complications such as abscess formation (12,13) (Fig 4).
Xanthogranulomatous pyelonephritis is a rare form of unilateral chronic pyelonephritis that is characterized by progressive parenchymal destruction and subsequent diffuse replacement by a macrophage-rich inflammatory infiltrate. The condition typically affects perimenopausal women with a history of recurrent urinary tract infections, diabetes mellitus, or obstructing calculi (14). Its diagnosis is challenging because clinical symptoms and laboratory findings are nonspecific. Typical CT findings include enlargement of the kidney, heterogeneous parenchymal enhancement or nonenhancement, hydronephrosis, abscesses, and obstructive staghorn-like calculi (14) (Fig 5). Extension of the inflammatory process into the perinephric space and retroperitoneum may simulate an infiltrative malignancy. In addition, the diagnosis of focal disease in the absence of obstructive calculi is difficult and may be made only at histopathologic examination. Diffuse xanthogranulomatous pyelonephritis is treated with an extended open nephrectomy, but focal disease is managed with a partial nephrectomy (14,15).
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Figure 5. Focal xanthogranulomatous pyelonephritis. Axial contrast-enhanced CT scan shows nonenhancement of the right kidney (arrow) with anterior extension of the infectious process (arrowhead).
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Hemangiopericytoma
Hemangiopericytoma is an uncommon retroperitoneal soft-tissue neoplasm that is hypothesized to arise from Zimmerman pericytes (16). The lesions most commonly occur in the extremities, particularly the thigh; the retroperitoneum is the second most common site. Hemangiopericytomas exhibit widely varying degrees of histologic and biologic aggressiveness (16,17).
Most retroperitoneal hemangiopericytomas appear as large, lobulated, well-circumscribed, hypervascular soft-tissue masses with foci of calcification and areas of necrosis (17). Displacement of the abdominopelvic viscera is more common than frank tissue invasion. Perinephric extension of retroperitoneal hemangiopericytomas frequently causes displacement of the kidneys. Preoperative embolization may be useful in some cases to reduce postoperative morbidity and achieve better resection; however, local and distant recurrences and metastases may occur. Ten-year survival rates of 47%–86% have been reported (17,18).
Leukemia
Leukemic involvement of the kidney is rare and may manifest at imaging as a diffusely infiltrating renal mass, a focal renal mass, or a perirenal mass (19,20). Granulocytic sarcomas (chloromas) are uncommon malignant neoplasms of granulocytic precursors that occur in up to 10% of patients with acute myelogenous leukemia. They are less commonly associated with acute lymphocytic leukemia. Renal chloromas appear as focal hypovascular soft-tissue masses in one or both kidneys (20). Perirenal leukemia occurs either as a result of perinephric extension of a renal lesion or, less commonly, as isolated leukemic involvement (Fig 6). The imaging findings are nonspecific, and a biopsy is required to obtain a definitive diagnosis (20).
Castleman Disease
Castleman disease is an uncommon idiopathic lymphoproliferative condition. Approximately 10%–15% of cases of Castleman disease occur in the abdomen and the retroperitoneum (21). Isolated perirenal lesions are uncommon. Castleman disease is classified as either hyaline-vascular subtype or plasma cell subtype on the basis of histologic findings and as either unicentric or multicentric on the basis of clinical manifestations (21). Multicentric Castleman disease is associated with Kaposi sarcoma from herpesvirus infection and is commonly seen in patients with acquired immunodeficiency syndrome. Multicentric Castleman disease is biologically more aggressive and is associated with a poor prognosis.
Imaging findings vary with the clinical and histologic subtype. Unicentric Castleman disease manifests as a well-defined, homogeneous, solitary intraabdominal mass with soft-tissue attenuation (22,23). Multicentric Castleman disease manifests in diffuse lymphadenopathy, hepatosplenomegaly, and ascites (21). Renal and perirenal involvement in the disease is rare (Fig 7). Findings of calcification, intense contrast enhancement, and central fibrosis favor a classification of hyaline-vascular subtype over that of plasma cell subtype (21,24).
Perirenal Hemangioma
Hemangiomas of the kidney are rare benign mesenchymal renal neoplasms that occur most often in the medulla and the renal sinus region (25). Most renal hemangiomas are solitary, small, and unilateral. Retroperitoneal and perirenal hemangiomas are extremely rare (26).
Imaging features of renal or perirenal hemangiomas are variable. The depiction of a soft-tissue mass with phleboliths, high signal intensity on T2-weighted MR images, and delayed or persistent contrast enhancement is suggestive of the diagnosis (25,27) (Fig 8). However, the imaging characteristics of renal and perirenal hemangiomas are nonspecific and may mimic those of transitional cell carcinoma or renal cell carcinoma (25,26).
Perirenal Lymphangioma
Renal and perirenal lymphangiomas are rare benign mesenchymal neoplasms that are histologically characterized by intercommunicating endothelium-lined spaces (28). The lesions may be unilateral or bilateral and diffuse or focal. At imaging, perirenal lymphangioma has the appearance of a uni- or multilocular cystic lesion with or without associated peripheral or septal enhancement (29,30) (Fig 9). Perirenal lymphangiomatosis is characterized by involvement of the entire perirenal mesenchyme, with bilateral perinephric cystic masses (31).
Renal Leiomyoma
Renal leiomyomas are rare benign mesenchymal neoplasms that mostly occur in asymptomatic adults. Perirenal leiomyomas are essentially exophytic renal tumors that arise from the renal capsule and extend into the perirenal space (32) (Fig 10). Imaging findings include a well-circumscribed soft-tissue mass that is supplied by renal capsular vessels (32). Larger leiomyomas are heterogeneous in appearance because of hemorrhage and cystic-myxoid degeneration (33).
Extragastrointestinal Stromal Tumor
The gastrointestinal stromal tumor (GIST) is a nonepithelial neoplasm that may originate from the c-KIT–positive interstitial cells of Cajal, the pacemaker cells of the gastrointestinal tract, where most GISTs are located. GISTs that occur outside the gastrointestinal tract are known as extragastrointestinal stromal tumors (34,35). Primary extragastrointestinal stromal tumors are extremely rare in perirenal locations; most are found in the omentum and mesentery (35). Perirenal extragastrointestinal stromal tumors appear as hypovascular soft-tissue masses (Fig 11). Their imaging features are nonspecific, and a biopsy is required for accurate diagnosis.
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Figure 11. Primary extragastrointestinal stromal tumor in the perinephric space. Axial contrast-enhanced CT scan demonstrates a soft-tissue mass (arrows) that has invaded the right kidney.
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Rindlike Soft-Tissue Masses
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Lymphoma
Perirenal involvement in lymphoma is usually due to contiguous spread from retroperitoneal or renal lymphoma. The occurrence of isolated lymphoma in the perirenal space is very unusual (<10% of cases of perirenal lymphoma) (36). Renal involvement in lymphoma occurs much more commonly in non-Hodgkin disease, with the majority of patients having intermediate or high-grade lymphomas of B-cell origin (37). However, in most cases of Hodgkin lymphoma with perirenal involvement, the renal parenchyma is not involved (38).
Renal lymphomas show several distinct imaging patterns: multiple masses, a solitary mass, a diffuse infiltrating renal mass, a rindlike soft-tissue thickening around the kidneys, and direct invasion from adjacent retroperitoneal lymphadenopathy (37). Isolated perirenal lymphoma appears as a uniformly attenuating rindlike soft-tissue mass around the kidney (Fig 12). The mass may invade or compress the kidney without significantly affecting renal function (37).
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Figure 12. Perirenal lymphoma. Axial contrast enhanced CT scan demonstrates a soft-tissue mass (arrows) that has encased the left kidney and renal vessels. RA = renal artery, RV = renal vein.
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Retroperitoneal Fibrosis
Retroperitoneal fibrosis is characterized by a proliferation of fibrous tissue around the aorta. The fibrous layer extends along the aorta through a plaquelike infiltrative soft-tissue process (39). Retroperitoneal fibrosis typically is localized to the distal abdominal (infrarenal) aorta and the common iliac arteries; involvement of the pelvis is uncommon (39). About two-thirds of cases are idiopathic (39). There is a 3:1 male-to-female preponderance among those affected by the disease (40).
Retroperitoneal fibrosis may be considered as an isolated disease or as part of a systemic syndrome known as multifocal fibrosclerosis, which also may include autoimmune pancreatitis, sclerosing cholangitis, scleroderma, Riedel thyroiditis, fibrotic pseudotumor of the orbit, and fibrosis involving multiple organ systems (40). Perirenal involvement may be secondary to extension from retroperitoneal fibrosis (41), may occur without associated retroperitoneal fibrosis (39,42), or may be one of various manifestations of multifocal fibrosclerosis (40,43).
Three stages of the disease have been described, ranging from chronic active inflammation to fibrous scarring (39). Typical imaging manifestations of perirenal fibrosis include a soft-tissue mass that envelops the kidneys without displacing them (40) (Fig 13). The T2-weighted MR signal intensity and dynamic enhancement characteristics depend on the stage of the disease. Whereas areas affected by active inflammation demonstrate high T2 signal intensity and early contrast enhancement, areas of fibrosis show low T2 signal intensity and delayed contrast enhancement (39). Perirenal fibrosis that occurs in association with retroperitoneal fibrosis or as part of multifocal fibrosclerosis is not difficult to detect at imaging. However, the imaging features of isolated perirenal fibrosis are nonspecific, and a biopsy may be required to achieve a definitive diagnosis (42).
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Figure 13. Perirenal fibrosis in a patient with scleroderma. Axial contrast-enhanced CT scan obtained during the corticomedullary phase depicts a rindlike soft-tissue layer surrounding both kidneys (arrows).
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Erdheim-Chester Disease
Erdheim-Chester disease is a rare form of systemic non–Langerhans cell histiocytosis of unknown etiology but with characteristic histologic and imaging findings (44). Histologically, Erdheim-Chester disease is defined by a mononuclear infiltrate consisting of lipid-laden, foamy histiocytes that stain positive for CD68 and negative for CD1a and S100 (45). Erdheim-Chester disease affects middle-aged individuals, without any specific sex predilection (46). Patients with the disease typically present with lower-extremity bone pain. Skeletal radiographic findings include bilateral symmetric metadiaphyseal cortical thickening, a coarsened trabecular pattern, and medullary sclerosis in the long bones of the appendicular skeleton, with sparing of the epiphyses and axial skeleton (47). Extraskeletal manifestations, which include central nervous system involvement, are seen in about half of those affected (46). Other organs in which involvement has been reported include the lungs, skin, kidneys, retroperitoneum, and heart (48).
Perirenal involvement in Erdheim-Chester disease characteristically is manifested as rindlike soft-tissue lesions surrounding the kidneys and ureters (45,49,50) (Fig 14). Severe compression of the renal parenchyma and ureters because of fibrous perinephritis leads to progressive renal failure (50). Percutaneous nephrostomy is difficult because of fibrous perinephritis. Ureteral stent placement and systemic corticosteroid therapy are the standard treatments until active inflammation resolves (49).
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Multiple Soft-Tissue Masses
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Perirenal Metastases
Metastases to the adrenal glands and kidneys are not uncommon. However, isolated perirenal metastases are rare. The perirenal space is an unusual but potentially significant site of metastases from lung cancer and other primary tumors such as malignant melanoma, breast carcinoma, and prostate cancer (51,52). Lung cancer shows a specific predilection for perirenal spread secondary to connections between the perirenal and mediastinal lymphatic vessels (1,51). At imaging, perirenal metastases frequently present as multiple soft-tissue masses around the kidney (52,53) (Fig 15).
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Figure 15. Perirenal metastases from bronchogenic carcinoma with mediastinal lymphadenopathy. Coronal contrast-enhanced CT scan shows multiple bilateral soft-tissue masses (arrowheads).
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Plasma Cell Neoplasms
Plasma cell neoplasms refers to a spectrum of malignant neoplasms that comprise multiple myeloma, plasmacytoma, and plasma cell leukemia (54). Although multiple myeloma originates in the bone marrow, it spreads to extramedullary sites in approximately 70% of patients (55,56). Plasmacytoma differs from multiple myeloma in that primary lesions may arise outside the bone marrow. Perirenal involvement in multiple myeloma manifests as multiple enhancing masses, whereas plasmacytoma manifests as a solitary enhancing mass with ill-defined margins (54,57,58) (Fig 16).
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Macroscopic Fat–containing Masses
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Angiomyolipoma
Angiomyolipoma is a benign unencapsulated mesenchymal neoplasm that is composed of smooth muscle cells, blood vessels, and adipose tissue in varying proportions (59). On the basis of a unifying histogenetic theory, angiomyolipomas are now considered within the category of neoplasms consisting of perivascular epithelioid cells (so-called PEComas) (60). Perirenal involvement usually results from perinephric extension of an angiomyolipoma of the kidney; primary perirenal angiomyolipoma arising from the perinephric mesenchyme is extremely rare (61). Primary perirenal angiomyolipomas have a strong female predilection and, like sporadic renal angiomyolipomas, are associated with an increased risk of hemorrhage.
Renal angiomyolipomas may occur either sporadically or as part of an autosomal dominant genetic disorder known as tuberous sclerosis complex. Sporadic renal angiomyolipomas are most often large, solitary, symptomatic tumors. Bilateral multiple renal angiomyolipomas occur in young patients (mean age, 25–35 years) with tuberous sclerosis complex. Because life-threatening hemorrhage and renal failure are common in those affected, angiomyolipomas are a significant cause of morbidity and mortality in adult patients with tuberous sclerosis complex (62). A subset of renal angiomyolipomas referred to as monotypic epithelioid angiomyolipomas may show aggressive biologic behavior including malignant transformation with regional or disseminated metastases, vascular invasion, local recurrence, and death (62). Epithelioid angiomyolipomas typically do not contain macroscopic fat and may not be distinguishable from renal cell carcinoma at imaging. A definitive diagnosis is based on a detailed histopathologic evaluation after resection or biopsy of the mass.
The imaging features of angiomyolipoma depend on the tumor size as well as the relative proportions of soft tissue, mature adipose tissue, and hypertrophied blood vessels within the tumor. Small angiomyolipomas appear homogeneous at cross-sectional imaging. Large angiomyolipomas are heterogeneous masses containing variable amounts of macroscopic fat, hypervascular soft tissue, and aneurysms (61,63) (Fig 17). The characteristic finding of macroscopic fat content is readily identified on CT scans or on frequency-selective fat-suppressed MR images (Fig 17). Renal angiomyolipoma with perinephric extension is the most common macroscopic fat–containing mass in the perirenal region. However, renal angiomyolipomas with minimal fat are indistinguishable from other renal soft-tissue masses, most notably renal cell carcinoma (64). Like renal angiomyolipoma, primary retroperitoneal angiomyolipoma demonstrates an increased propensity to bleed; the risk of hemorrhage correlates with the size of the tumor (59). Renal angiomyolipoma with perinephric extension must be distinguished from a primary retroperitoneal (perirenal) liposarcoma. The presence of a well-defined renal parenchymal defect and intra- lesional aneurysms are suggestive of a diagnosis of angiomyolipoma of the kidney rather than a liposarcoma (63) (Fig 18).
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Figure 18. Renal angiomyolipoma. Axial contrast-enhanced CT scan demonstrates an exophytic fat-containing mass (arrows) that has arisen from the right kidney. Note the characteristic parenchymal defect (arrowhead).
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Liposarcoma
Liposarcoma is the most common primary retroperitoneal malignant neoplasm. There are five histologic subtypes: well differentiated, myxoid, round cell, pleomorphic, and dedifferentiated (65). Retroperitoneal liposarcomas are typically well differentiated or dedifferentiated (with frequent genetic amplification of the 12q13–15 region) and are commonly large at presentation (66). Dedifferentiation also is commonly seen (10%–15% of cases) because of the long latent period before diagnosis (66). There is no specific sex predilection, and the peak occurrence is in the 6th and 7th decades of life (between 51 and 70 years of age).
Imaging findings commonly reflect histologic features. Well-differentiated liposarcomas have a predominant macroscopic fat component, whereas round cell and pleomorphic types appear as soft-tissue masses (Fig 19) (65,67). Myxoid liposarcoma appears cystic, with reticular enhancement after contrast material is administered. The surgical resection of retroperitoneal liposarcomas is technically difficult because of their large size, and a high rate of local recurrence (90%) has been reported (66). Well-differentiated liposarcomas are biologically less aggressive; the myxoid subtype has an intermediate prognosis; and the round cell and pleomorphic subtypes are considered high grade, with high rates of local recurrence and metastasis (66,67).
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Figure 19. Perirenal liposarcoma. Axial contrast-enhanced CT scan shows a well-circumscribed mass with predominant fat content (arrows). The mass is localized in the right perirenal space.
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Extramedullary Hematopoiesis
Extramedullary hematopoiesis occurs when hematopoietic tissue develops outside primary (medullary) sites. Such occurrences may be associated with hemolytic anemia, hemoglobinopathies, primary and secondary myelofibrosis, leukemia, lymphoma, or skeletal metastases (68). The most common sites of extramedullary hematopoiesis are the liver, spleen, and paraspinal regions of the thorax, a distribution that reflects the various levels of hematopoietic activity in these sites during embryogenesis (69). Extramedullary hematopoiesis also has been reported in various other sites, including lymph nodes, mediastinum, central nervous system, pleura and lungs, heart, gastrointestinal tract, and kidneys (69,70).
Perirenal involvement in extramedullary hematopoiesis is uncommon and manifests one of two distinct imaging patterns: a diffuse infiltrative process surrounding the kidneys, or soft-tissue masses intermixed with macroscopic fat (68,70,71). On CT scans, extramedullary hematopoiesis appears as a hypovascular soft-tissue mass with or without associated macroscopic fat (Fig 20). On T2-weighted MR images, the soft-tissue component has a characteristic low signal intensity because of its hemosiderin content (71).
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Figure 20. Extramedullary hematopoiesis. Axial contrast-enhanced CT scan shows large bilateral perirenal heterogeneous masses (arrows) with predominant fatty components (arrowheads).
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Perirenal extramedullary hematopoiesis poses particular diagnostic difficulty when it occurs in unusual sites because the imaging findings may resemble those of other neoplasms. The diffuse infiltrative type typically mimics lymphoma, whereas the macroscopic fat–containing type resembles other fatty neoplasms (71). The diagnosis therefore is usually established at histopathologic analysis.
Extraadrenal Myelolipoma
Myelolipomas are rare benign monoclonal neoplasms composed of an admixture of mature adipose tissue and normal hematopoietic cells (72). Four distinct types of myelolipomas have been described: isolated adrenal myelolipoma, adrenal myelolipoma with hemorrhage, extraadrenal myelolipoma, and myelolipoma associated with other adrenal disease (73). The adrenal gland is the most common target site of myelolipoma. Extraadrenal myelolipomas are distinctly rare but their occurrence has been described in the retroperitoneum, stomach, liver, lung, presacral area, and mediastinum. Extraadrenal myelolipoma shows a particular predilection for involving the perirenal and presacral regions (74). On average, patients with extraadrenal myelolipoma are a decade older (mean age, 64 years) than those with adrenal myelolipoma (mean age, 55 years). Extraadrenal lesions seem slightly more common in females (75).
Imaging findings depend on the proportions of the adipose and hematopoietic tissue contents. Myelolipomas typically appear heterogeneous, with both hypervascular soft-tissue and macroscopic fat components depicted on CT scans and MR images (75,76). The presence of macroscopic fat is a characteristic finding in myelolipoma (Fig 21) (75). Whereas adrenal myelolipomas are readily diagnosed on the basis of their imaging appearance and location, extraadrenal myelolipomas may be mistaken for other fat-containing masses. The diagnosis is usually established at histopathologic analysis.
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Figure 21. Perirenal myelolipoma. Axial contrast-enhanced CT scan demonstrates an indistinct fatty mass (arrowheads) that indents the left kidney and merges with the adjacent retroperitoneal fat.
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Summary
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A wide spectrum of neoplastic and nonneoplastic proliferative conditions may involve the perirenal space either in isolation or as part of a systemic disease process. Perirenal localization may be the predominant or the only site of involvement in a systemic condition. Although some tumors and pseudotumors of the perirenal space (eg, angiomyolipoma, hemangioma, and lymphangioma) have characteristic imaging findings that permit their diagnosis, a biopsy and histopathologic evaluation are required in most cases to establish a definitive diagnosis. Nevertheless, familiarity with the spectrum of imaging features of perirenal tumors and pseudotumors may facilitate accurate diagnosis and timely treatment.
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Footnotes
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Abbreviations: GIST = gastrointestinal stromal tumor
2 Current address: Department of Diagnostic and Interventional Imaging at the University of Texas Health Science Center, Houston, Texas. 
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Abstract
LEARNING OBJECTIVES FOR TEST...
